![]() ![]() That way, a smaller spread in the process gives a narrower control zone, while a greater spread gives a wider control zone (see Fig. The correct way is to let the control limits adapt to the process. That also means that 0.27% of the outcome is not included in the normal distribution curve. These six account for 99.73% of the actual result. The normal distribution curve is thus derived from one standard deviation and consists of six of them. If you had gone on making measurements you could have plotted the curve, but now you have calculated it instead (see Fig. To calculate the normal distribution spread, you simply multiply the standard deviation by 6 to get the total width of the normal distribution curve. ![]() Instead you can calculate the spread using the standard deviation (see below). This means that you do not need to measure hundreds of components to find out how much the machine or process is varying. This distance constitutes one standard deviation (see Fig. ![]() The procedure is that you measure the distance from the mean value (highest point on the hump) to the point where the curve changes direction and starts to swing outward. This is a statistical function used to calculate the normal distribution curve, for example. If a point falls outside a control limit on the R graph, the spread of the process has changed (see Fig. If a point falls outside a control limit on the X graph, the position of the process has changed (see Fig. This means, in principle, that you have no reason to react until the control chart signals certain behaviour.Ī commonly used control graph is the XR graph, where the position and spread of the process are monitored with the help of sub groups and control limits. The function of control limits is to centre the process on the target value, which is usually the same as the middle of the tolerance width, and to show where the limit of a stable process lies. ![]() They have nothing to do with tolerance limits, because they are designed to call your attention when the process changes its behaviour.Īn important principle is that control limits are used along with the mean value on the control graph to control the process, unlike tolerance limits, which are used along with individual measurements to determine whether a given part meets specifications or not. Control limits are an important aspect of statistical process control. ![]()
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